Portable gas spectroscopy may useful in settings that include indoor or confined space air monitoring and breath analysis. Current technologies include either small devices that detect a limited number of gas types (such as for common roadside alcohol breathalyzer tests) or may lack sensitivity. Larger devices may be able to scan for a broader number of gas types, but their size and power consumption may make them undesirable in many environments.
As an overview of the technology, electro-magnetic waves in the millimeter and submillimeter wave frequency ranges can be used for fast scan rotational spectroscopy to detect gas molecules and measure their concentrations. FIG. 1 shows the principle behind rotational spectroscopy, in which a transmitter 100 such as a VDI transmitter radiates electromagnetic waves 105 within a certain frequency range into an absorption cell 110 containing gas molecules 115. The waves change the rotational state of gas molecules and a part of the wave power is lost in this interaction. These results in a drop of power picked up 125 at a receiver (RX) 120 such as a VDI receiver or a spike. By sweeping the transmitter (TX) 100 output frequency, a sweep controlled by a computer (not shown), the receiver 120 may measure the frequency response of the absorption cell 110. As shown in the frequency vs power absorption graph 140, looking at the frequencies at which the waves are absorbed and the depth of absorption line, the presence of a particular molecule and its concentration may be determined.
Rotational spectrometers use a frequency multiplier chain driven from a signal generated using a synthesizer to generate the transmitter signal, and a diode based sub-harmonic mixer followed by an amplifier and a diode amplitude detector to measure the received signal amplitude. The frequency multiplier chain, mixer and diode may be fabricated using compound-semiconductor technologies. Rotational spectroscopy at 240-250 GHz has been demonstrated using SiGe heterojunction bipolar transistor (HBT) based radio frequency (RF) front-ends for transmission and reception and a spectrometer that measures 200-300 GHz frequency waves uses III-V technology (obtained by combining group III elements (Al, Ga, In) with group V elements (N, P, As, Sb)) but is both expensive and bulky.